In the field of X-ray imaging, for example, in computed tomography, angiography or radiography, counting direct-conversion X-ray detectors can be used. The X-ray radiation or the photons can be converted into electrical pulses via a suitable sensor. CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs, for example, or others can be used as the sensor material. The electric pulses are evaluated by an evaluating electronic system, for example an integrated circuit (Application Specific Integrated Circuit, ASIC).
In counting X-ray detectors, incident X-ray radiation is measured by counting the electric pulses which are triggered by the absorption of X-ray photons in the detector material. The size of the electric pulses is typically proportional to the energy of the absorbed X-ray photon. By this, spectral information can be extracted by comparison of the size of the electric pulse with a threshold value. Only those electric pulses which exceed the threshold are counted as an event. A photon-counting detector element, for example, a subpixel or a macropixel formed from a plurality of subpixels grouped together can have a plurality of threshold values and a register associated with the threshold values. The threshold values can be associated with different photon energies. The incident spectrum can thus be scanned in a plurality of energy intervals. X-ray recordings with spectral information can thus be provided.
The characteristic absorption spectrum or the different absorption behavior of different materials can be used for material differentiation. This can typically be used in the reconstruction of computed tomography recordings with a plurality of base materials, for example, water, iodine, bone, gadolinium or gold. The differentiation of a plurality of base materials requires an equivalent number of independent measurements. The measurements can be carried out with different energy channels or threshold values. Thus, a plurality of threshold values or energy channels are needed in order to be able to differentiate the materials within the examination object. Apart therefrom, independent measurements can also enable a weighted addition of the counting events in different energy channels, which enables an improved contrast for monochromatic X-ray images. Counting X-ray detectors can therefore advantageously be suitable, in particular, for recording slice images, for example, with a computed tomography device or a C-arm angiography system.
Known implementations of a plurality of energy channels realize a smallest possible implementation of the digital-analogue converter within a detector element, for example, with a current-controlled digital-analogue converter (current steering DAC), an R2R network, a segmented digital-analogue converter or a digital-analogue converter with pulse width modulation (PWM) and a low pass filter. Alternatively, digital-analogue converters can be used which are usable globally for the whole matrix of detector elements, wherein one digital-analogue converter is used per energy channel. In addition to the supply of a voltage from the global digital-analogue converter, each detector element has an additional smaller digital-analogue converter, wherein this smaller digital-analogue converter enables a fine setting of the threshold value. The known implementations have a direct proportional relationship between the number of energy channels and the area of the digital-analogue converter and the power consumption of the digital-analogue converter.
A plurality of, or additional, energy channels are associated with an increased space requirement, an increased power consumption and a higher required bandwidth for readout. The energy thresholds require a settable reference voltage for the threshold value, a comparator and a register. The increased space requirement limits the size of the detector elements. The size of the detector elements must be adapted to the space requirement of the energy channels. The smaller the size of the detector elements, the better the spatial resolution of the medical device can be.